The present invention relates to an offshore unit structure. When building up an offshore unit structure on a sea bottom soft ground, in order to prevent the structure from being deformed by gravitational load of the structure or by horizontal load applied thereto, a cement type solidifying agent is impregnated into and mixed with such soft ground to construct a unit-volume reformed ground, and the desired structure is built upon such reformed ground.
The present invention also can be applied to a unit structure used in prospecting for or production of oil in the sea.
For prospecting for or production of oil in the sea such as the Arctic Ocean, it is required to build a platform or artificial island at every spot well head.
In the Arctic Ocean, however, the structure built upon the sea bottom is subject to a heavy horizontal load by ice, and even a slight slide of the structure in the horizontal direction due to such ice load or an earthquake may cause serious damage to the pipes and other members which run deep into the sea bottom ground, so that the platform or artificial island must be constructed firm and steadfast. In the case of the conventional construction methods, it costs a great deal to build such a structure, and the built structure has a safety problem. Further, the conventional construction of the structure requires too much time, and therefore it is hardly possible to build the structure during the short summer in the Arctic.